PR libstdc++/61947 * include/std/tuple (_Head_base): Use allocator_arg_t parameters to disambiguate unary constructors. (_Tuple_impl): Pass allocator_arg_t arguments. * testsuite/20_util/tuple/61947.cc: New. * testsuite/20_util/uses_allocator/cons_neg.cc: Adjust dg-error line. From-SVN: r213221